Anyway, I just got passed down 2 full-grown fish, and the LF won't come out of hiding, and the Pacu is a little more bold, but not much. I was thinking I need to add some dither fish, but what could I add that the Pacu wouldn't eat? (he's about 11" long, about 7-8" tall)

Silver Dollars? Zebra or Giant Danios?

I have several rocks, a few plants, and the LF normal hides down in the roots of a ceramic tree stump.

Welcome to the forum, Sean! I don't know anything about either fish except for one tiny little thing. Your pacu is not an adult yet, as he will get much, much bigger and your 75 won't be enough of a tank for him in the future. I've seen many people write about their their emotional attachment to these fish so watch out...a 200 gl may be in your future! Once again, welcome to the forum, glad you joined us

Any updates? Pacus are among my favorite fish but Kymmie is right, they will need a much larger aquarium than 75 gallons!

As for dither fish, I definitely wouldn't go with Danios as they're likely to be eaten. Pacu's generally aren't aggressive unless they run out of territory, but they are known to eat fish small enough to fit into their mouths. Tinfoil Barbs would be a good choice but again, you're looking at a much larger tank requirement.